Under normal circumstances, a Global Navigation Satellite System receiver (GPS or a receiver configured to operate in combination with other systems such as GLONASS) requires a minimum of four satellites to obtain a position estimate. Due to obstructions such as buildings and trees, satellite (SV) visibility, geometry, and signal quality may degrade. In these conditions, the accuracy of GPS positions will typically decrease, until they are unavailable.
This happens when there are either insufficient satellites (three or less), or the conditions become so bad that they fall below some threshold set by the user.
If one of the four GPS variables (three position axes, x, y, and z, and receiver clock error) is known however, the solution process can be modified to solve for only three variables—the minimum number of required satellites becomes only three, and in the case of bad conditions, the extra constraint may allow better accuracy in the other three measurements. Commonly used constraints and measurement methods include acceleration (accelerometer), heading (gyroscope), and altitude (barometric altimeter).
Herein is described the use of a barometer to augment obtaining improved position fixes in GNSS receivers, as well as improving the availability thereof under degrading conditions caused by multipath, limited satellite visibility, and poor satellite geometric conditions.
Experiments with suitable GNSS receivers and a barometric altimeter have shown that improvements in the yield of acceptable position fixes can be obtained. In all cases, the prototype results were much more accurate than those obtained using an unaided off-the-shelf GPS receiver. With differential barometry, useful lifetime of the constraint can be extended to entire data collection sessions, with submeter accuracy achievable for averaged features.